lib/iomgr/timer_manager.cc

//
//
// Copyright 2017 gRPC authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
//

#include "src/core/lib/iomgr/timer_manager.h"

#include <grpc/support/alloc.h>
#include <grpc/support/port_platform.h>
#include <inttypes.h>

#include "absl/log/check.h"
#include "absl/log/log.h"
#include "src/core/lib/debug/trace.h"
#include "src/core/lib/iomgr/timer.h"
#include "src/core/util/crash.h"
#include "src/core/util/thd.h"

struct completed_thread {
  grpc_core::Thread thd;
  completed_thread* next;
};

// global mutex
static gpr_mu g_mu;
// are we multi-threaded
static bool g_threaded;
// should we start multi-threaded
static bool g_start_threaded = true;
// cv to wait until a thread is needed
static gpr_cv g_cv_wait;
// cv for notification when threading ends
static gpr_cv g_cv_shutdown;
// number of threads in the system
static int g_thread_count;
// number of threads sitting around waiting
static int g_waiter_count;
// linked list of threads that have completed (and need joining)
static completed_thread* g_completed_threads;
// was the manager kicked by the timer system
static bool g_kicked;
// is there a thread waiting until the next timer should fire?
static bool g_has_timed_waiter;
// the deadline of the current timed waiter thread (only relevant if
// g_has_timed_waiter is true)
static grpc_core::Timestamp g_timed_waiter_deadline;
// generation counter to track which thread is waiting for the next timer
static uint64_t g_timed_waiter_generation;
// number of timer wakeups
static uint64_t g_wakeups;

static void timer_thread(void* completed_thread_ptr);

static void gc_completed_threads(void) {
  if (g_completed_threads != nullptr) {
    completed_thread* to_gc = g_completed_threads;
    g_completed_threads = nullptr;
    gpr_mu_unlock(&g_mu);
    while (to_gc != nullptr) {
      to_gc->thd.Join();
      completed_thread* next = to_gc->next;
      gpr_free(to_gc);
      to_gc = next;
    }
    gpr_mu_lock(&g_mu);
  }
}

static void start_timer_thread_and_unlock(void) {
  CHECK(g_threaded);
  ++g_waiter_count;
  ++g_thread_count;
  gpr_mu_unlock(&g_mu);
  GRPC_TRACE_LOG(timer_check, INFO) << "Spawn timer thread";
  completed_thread* ct =
      static_cast<completed_thread*>(gpr_malloc(sizeof(*ct)));
  ct->thd = grpc_core::Thread("grpc_global_timer", timer_thread, ct);
  ct->thd.Start();
}

void grpc_timer_manager_tick() {
  grpc_core::ExecCtx exec_ctx;
  grpc_timer_check(nullptr);
}

static void run_some_timers() {
  // In the case of timers, the ExecCtx for the thread is declared
  // in the timer thread itself, but this is the point where we
  // could start seeing application-level callbacks. No need to
  // create a new ExecCtx, though, since there already is one and it is
  // flushed (but not destructed) in this function itself
  grpc_core::ApplicationCallbackExecCtx callback_exec_ctx(
      GRPC_APP_CALLBACK_EXEC_CTX_FLAG_IS_INTERNAL_THREAD);

  // if there's something to execute...
  gpr_mu_lock(&g_mu);
  // remove a waiter from the pool, and start another thread if necessary
  --g_waiter_count;
  if (g_waiter_count == 0 && g_threaded) {
    // The number of timer threads is always increasing until all the threads
    // are stopped. In rare cases, if a large number of timers fire
    // simultaneously, we may end up using a large number of threads.
    start_timer_thread_and_unlock();
  } else {
    // if there's no thread waiting with a timeout, kick an existing untimed
    // waiter so that the next deadline is not missed
    if (!g_has_timed_waiter) {
      GRPC_TRACE_LOG(timer_check, INFO) << "kick untimed waiter";
      gpr_cv_signal(&g_cv_wait);
    }
    gpr_mu_unlock(&g_mu);
  }
  // without our lock, flush the exec_ctx
  GRPC_TRACE_LOG(timer_check, INFO) << "flush exec_ctx";
  grpc_core::ExecCtx::Get()->Flush();
  gpr_mu_lock(&g_mu);
  // garbage collect any threads that are dead
  gc_completed_threads();
  // get ready to wait again
  ++g_waiter_count;
  gpr_mu_unlock(&g_mu);
}

// wait until 'next' (or forever if there is already a timed waiter in the pool)
// returns true if the thread should continue executing (false if it should
// shutdown)
static bool wait_until(grpc_core::Timestamp next) {
  gpr_mu_lock(&g_mu);
  // if we're not threaded anymore, leave
  if (!g_threaded) {
    gpr_mu_unlock(&g_mu);
    return false;
  }

  // If g_kicked is true at this point, it means there was a kick from the timer
  // system that the timer-manager threads here missed. We cannot trust 'next'
  // here any longer (since there might be an earlier deadline). So if g_kicked
  // is true at this point, we should quickly exit this and get the next
  // deadline from the timer system

  if (!g_kicked) {
    // if there's no timed waiter, we should become one: that waiter waits
    // only until the next timer should expire. All other timers wait forever
    //
    // 'g_timed_waiter_generation' is a global generation counter. The idea here
    // is that the thread becoming a timed-waiter increments and stores this
    // global counter locally in 'my_timed_waiter_generation' before going to
    // sleep. After waking up, if my_timed_waiter_generation ==
    // g_timed_waiter_generation, it can be sure that it was the timed_waiter
    // thread (and that no other thread took over while this was asleep)
    //
    // Initialize my_timed_waiter_generation to some value that is NOT equal to
    // g_timed_waiter_generation
    uint64_t my_timed_waiter_generation = g_timed_waiter_generation - 1;

    // If there's no timed waiter, we should become one: that waiter waits only
    // until the next timer should expire. All other timer threads wait forever
    // unless their 'next' is earlier than the current timed-waiter's deadline
    // (in which case the thread with earlier 'next' takes over as the new timed
    // waiter)
    if (next != grpc_core::Timestamp::InfFuture()) {
      if (!g_has_timed_waiter || (next < g_timed_waiter_deadline)) {
        my_timed_waiter_generation = ++g_timed_waiter_generation;
        g_has_timed_waiter = true;
        g_timed_waiter_deadline = next;

        if (GRPC_TRACE_FLAG_ENABLED(timer_check)) {
          grpc_core::Duration wait_time = next - grpc_core::Timestamp::Now();
          LOG(INFO) << "sleep for a " << wait_time.millis() << " milliseconds";
        }
      } else {  // g_timed_waiter == true && next >= g_timed_waiter_deadline
        next = grpc_core::Timestamp::InfFuture();
      }
    }

    if (GRPC_TRACE_FLAG_ENABLED(timer_check) &&
        next == grpc_core::Timestamp::InfFuture()) {
      LOG(INFO) << "sleep until kicked";
    }

    gpr_cv_wait(&g_cv_wait, &g_mu, next.as_timespec(GPR_CLOCK_MONOTONIC));

    GRPC_TRACE_LOG(timer_check, INFO)
        << "wait ended: was_timed:"
        << (my_timed_waiter_generation == g_timed_waiter_generation)
        << " kicked:" << g_kicked;
    // if this was the timed waiter, then we need to check timers, and flag
    // that there's now no timed waiter... we'll look for a replacement if
    // there's work to do after checking timers (code above)
    if (my_timed_waiter_generation == g_timed_waiter_generation) {
      ++g_wakeups;
      g_has_timed_waiter = false;
      g_timed_waiter_deadline = grpc_core::Timestamp::InfFuture();
    }
  }

  // if this was a kick from the timer system, consume it (and don't stop
  // this thread yet)
  if (g_kicked) {
    grpc_timer_consume_kick();
    g_kicked = false;
  }

  gpr_mu_unlock(&g_mu);
  return true;
}

static void timer_main_loop() {
  for (;;) {
    grpc_core::Timestamp next = grpc_core::Timestamp::InfFuture();
    grpc_core::ExecCtx::Get()->InvalidateNow();

    // check timer state, updates next to the next time to run a check
    switch (grpc_timer_check(&next)) {
      case GRPC_TIMERS_FIRED:
        run_some_timers();
        break;
      case GRPC_TIMERS_NOT_CHECKED:
        // This case only happens under contention, meaning more than one timer
        // manager thread checked timers concurrently.

        // If that happens, we're guaranteed that some other thread has just
        // checked timers, and this will avalanche into some other thread seeing
        // empty timers and doing a timed sleep.

        // Consequently, we can just sleep forever here and be happy at some
        // saved wakeup cycles.
        GRPC_TRACE_LOG(timer_check, INFO)
            << "timers not checked: expect another thread to";
        next = grpc_core::Timestamp::InfFuture();
        ABSL_FALLTHROUGH_INTENDED;
      case GRPC_TIMERS_CHECKED_AND_EMPTY:
        if (!wait_until(next)) {
          return;
        }
        break;
    }
  }
}

static void timer_thread_cleanup(completed_thread* ct) {
  gpr_mu_lock(&g_mu);
  // terminate the thread: drop the waiter count, thread count, and let whomever
  // stopped the threading stuff know that we're done
  --g_waiter_count;
  --g_thread_count;
  if (0 == g_thread_count) {
    gpr_cv_signal(&g_cv_shutdown);
  }
  ct->next = g_completed_threads;
  g_completed_threads = ct;
  gpr_mu_unlock(&g_mu);
  GRPC_TRACE_LOG(timer_check, INFO) << "End timer thread";
}

static void timer_thread(void* completed_thread_ptr) {
  // this threads exec_ctx: we try to run things through to completion here
  // since it's easy to spin up new threads
  grpc_core::ExecCtx exec_ctx(GRPC_EXEC_CTX_FLAG_IS_INTERNAL_THREAD);
  timer_main_loop();

  timer_thread_cleanup(static_cast<completed_thread*>(completed_thread_ptr));
}

static void start_threads(void) {
  gpr_mu_lock(&g_mu);
  if (!g_threaded) {
    g_threaded = true;
    start_timer_thread_and_unlock();
  } else {
    gpr_mu_unlock(&g_mu);
  }
}

void grpc_timer_manager_init(void) {
  gpr_mu_init(&g_mu);
  gpr_cv_init(&g_cv_wait);
  gpr_cv_init(&g_cv_shutdown);
  g_threaded = false;
  g_thread_count = 0;
  g_waiter_count = 0;
  g_completed_threads = nullptr;

  g_has_timed_waiter = false;
  g_timed_waiter_deadline = grpc_core::Timestamp::InfFuture();

  if (g_start_threaded) start_threads();
}

static void stop_threads(void) {
  gpr_mu_lock(&g_mu);
  GRPC_TRACE_LOG(timer_check, INFO)
      << "stop timer threads: threaded=" << g_threaded;
  if (g_threaded) {
    g_threaded = false;
    gpr_cv_broadcast(&g_cv_wait);
    GRPC_TRACE_LOG(timer_check, INFO)
        << "num timer threads: " << g_thread_count;
    while (g_thread_count > 0) {
      gpr_cv_wait(&g_cv_shutdown, &g_mu, gpr_inf_future(GPR_CLOCK_MONOTONIC));
      GRPC_TRACE_LOG(timer_check, INFO)
          << "num timer threads: " << g_thread_count;
      gc_completed_threads();
    }
  }
  g_wakeups = 0;
  gpr_mu_unlock(&g_mu);
}

void grpc_timer_manager_shutdown(void) {
  stop_threads();

  gpr_mu_destroy(&g_mu);
  gpr_cv_destroy(&g_cv_wait);
  gpr_cv_destroy(&g_cv_shutdown);
}

void grpc_timer_manager_set_threading(bool enabled) {
  if (enabled) {
    start_threads();
  } else {
    stop_threads();
  }
}

void grpc_timer_manager_set_start_threaded(bool enabled) {
  g_start_threaded = enabled;
}

void grpc_kick_poller(void) {
  gpr_mu_lock(&g_mu);
  g_kicked = true;
  g_has_timed_waiter = false;
  g_timed_waiter_deadline = grpc_core::Timestamp::InfFuture();
  ++g_timed_waiter_generation;
  gpr_cv_signal(&g_cv_wait);
  gpr_mu_unlock(&g_mu);
}

uint64_t grpc_timer_manager_get_wakeups_testonly(void) { return g_wakeups; }